Annual = Not good

During the past decade research studies have found that annual doses of vitamin D do not provide much benefit.
There are far fewer studies publishing results with annual doses.
Mostly the studies being published since 2010 are ones which were started many years ago.
We are unaware of any new Clinical Trials using annual dosesSometimes annual doses have actually ended up causing problems - such as more falls.
Imagine the consequences of taking a years worth of any food, supplement, or drug in a single day:
such as: salt, water, iodine, aspirin, sugar, wheat, vitamin A, etc, etc.
Vitamin D is perhaps the only item for which taking the annual amount in a single day will not cause a call to 911 or immediate death

Quarterly = OK for some

Quarterly doses of vitamin D results in significant variation in blood levels of vitamin D - which stresses the body.
It appears, however, that the benefits of the vitamin D out-weigh the stresses of quarterly dosing.

Monthly = OK for most

The half-life for vitamin D for most people is approximately 1 month.
So, for most people, taking vitamin D monthly will result in a blood level of vitamin D which drops perhaps 30% by the end of the month
This variation results in some stress on the body.
Evolution has created bodies which typically experience even larger summer-winter changes in vitamin D levels
Note: there is some thought that the body needs to have variation in the vitamin D level - so should not keep it at a constant level all year long

The half life of vitamin D appears to be about a week for high level of vitamin D

Thus monthly doses of vitamin D appears to be NOT appropriate for people with vitamin D levels higher than 50ng/ml

Weekly = Great for most

Virtually everyone can take vitamin D weekly, say on Saturday, without any problems.
Note: Should probably take vitamin D co-factors more frequently.
The body cannot take a whole weeks worth at a time and the body does not store some of the co-factors very well.

Infrequent dosing provides higher Vitamin D gradient

Higher gradient (steeper slide) resulting from infrequent dosing results in more Vitamin D getting into the cells.Note: Virtually all cells of the body can utilize/activate Vitamin D, no liver or kidney needed

Daily: Can be this often, but it is not necessary.

Since most people take their pills on a daily basis (including co-factors) vitamin D will typically be taken daily.

Half-Life appears to vary with amount of vitamin D

Half-life = how long till half of the substance is gone (from the blood)
For people with a very low level of 10 nanograms, the response will fall by half in about 10 weeks
For people with a very high level of 50 nanograms, the response will fall by half in about 1 week

Notion of Vitamin D benefit vs dosing frequency for > 50 nanograms

Note: Have not yet seen any study of vitamin D half-life. Would like to see one.